1. Field of the Invention
The present invention relates to a submarine optical cable, an optical fiber unit employed in the submarine optical cable, and a method of making the optical fiber unit.
2. Related Background Art
Submarine optical cables have been developed substantially in parallel with land-based cables since the mid-1970s when the reduction of loss in optical fibers had advanced and enhanced the feasibility of optical fiber communications. Optical fibers have quite excellent characteristics as a transmission medium, such as low loss, wide band, small diameter, light weight, no induction, and no crosstalk. On the other hand, the optical fibers have not only brittleness and breakableness inherent in glass materials but also a demerit that optical loss is likely to increase due to bending or external forces.
The structure of a submarine optical cable needs to reflect its purpose of use, environment of use, method of installation, and method of repair, which should be grasped beforehand. Namely, it is required for the submarine optical cable to endure external forces such as tension, bending, abrasion, and the like upon installing the cable and collecting the cable at the time of repairing, and maintain stable characteristics over a long period of 25 years or more under a submarine environment as deep as 8,000 m (at a water pressure of 800 atmospheres and a water temperature of about 3xc2x0 C.). When a transpacific system is taken into consideration, the maximum cable length extends to about 10,000 km. If seawater should enter the cable, the strength and transmission characteristics of optical fibers will deteriorate. Therefore, it is required to prevent seawater from entering the cable. Further, it is necessary for the cable to have an economical structure which can easily be made long while satisfying various conditions mentioned above.
A first conventional example of submarine optical cable used for the above-mentioned purpose is disclosed in Japanese Patent Application Laid-Open No. HEI 10-170775. As shown in FIG. 1, the first conventional example comprises an optical fiber unit B and an outer sheath A with a predetermined structure disposed at the outer periphery of the optical fiber unit B. The optical fiber unit B is constituted by a center steel wire D, coated optical fibers C stranded about the center steel wire D, and a UV-curable resin E securing the coated optical fibers C to the center steel wire D. For restraining microbend from occurring due to thermal expansion/contraction upon changes in temperature, each coated optical fiber C comprises three resin layers disposed at the outer periphery of an optical fiber. The first layer is made of a UV-curable resin having a lower Young""s modulus, the second layer is made of a UV-curable resin having a higher Young""s modulus, and the third layer is made of a nylon resin having a higher Young""s modulus.
On the other hand, as a second conventional example, a submarine optical cable having a metal tube is disclosed in IWCS Proceedings 1996, pp. 8-9: New Submarine Cable Design for Long Haul, High Bit Rate Systems. As shown in FIG. 2, the optical fiber unit J of the second conventional example comprises a metal tube F in which coated optical fibers G and a jelly-like resin H are introduced. The submarine optical cable of the second conventional example comprises an outer sheath I with a predetermined structure disposed at the outer periphery of the optical fiber unit J.
As a result of studies concerning the conventional submarine optical cables, the inventors have found the following problems.
Along with the introduction of WDM communications, optical-fibers having a large mode field diameter or effective area have become necessary for submarine optical cables as well. In such an optical fiber, however, power distribution widely spreads into its cladding, so that the radiation mode may change as the optical fiber is bent, thus increasing loss. Hence, in the optical fiber unit B (FIG. 1) in which the coated optical fibers C wound about the outer periphery of the tension member D are secured with the UV-curable resin E as in the case of the first conventional example, bending loss may increase.
On the other hand, in the second conventional example comprising an optical fiber unit in which the coated optical fibers G and the jelly-like resin H are introduced into the metal tube F, it is necessary that the coated optical fibers G and the jelly-like resin H be introduced into the metal tube F while the latter is being made by longitudinally welding a metal tape. In this manufacturing method, there is a limit to the welding speed. As a consequence, the cost of manufacture becomes quite high, and the high temperature of welding heat is likely to affect the coated optical fibers, thus making it hard to manufacture the cable stably.
It is an object of the present invention to overcome such problems and provide a wide-band, low-loss, submarine optical cable having a structure excellent in productivity, an optical fiber unit employed in the submarine optical cable, and a method of making the optical fiber unit.
The optical fiber unit according to the present invention is employed in a submarine optical cable and comprises a loose structure which accommodates one or more coated optical fibers with a margin.
Specifically, the optical fiber unit according to a first embodiment has a plastic support provided with a space for accommodating one or more coated optical fibers. The space of this plastic support extends along a predetermined axis, and a gap between an inner wall face of the plastic support defining the space and the coated optical fibers accommodated within the space is filled with a waterproof jelly- or gel-like resin. In particular, in the optical fiber unit according to the first embodiment, one or more rigid compression members are embedded along the space in a plastic region between the inner wall face of the plastic support and an outer wall face of the plastic support defining an outer shape of the plastic support.
The optical fiber unit according to the present invention comprises a loose structure which movably accommodates coated optical fibers within a space of a plastic support. As a consequence, lateral pressures would not directly apply to the coated optical fibers, whereby loss can be kept from increasing due to microbend.
Also, since the compression members embedded in the plastic support according to the present invention are directly in contact with the plastic support, they restrict the contraction of the plastic support caused by changes in temperature or the contraction of the plastic immediately after its extrusion. Therefore, the coated optical fibers accommodated within the plastic support would not yield a surplus length more than necessary with respect to the plastic support. If the surplus length of the coated optical fibers with respect to the plastic support is too long, then the surplus length may locally concentrate as temperature repeatedly changes, thereby increasing the transmission loss. On the other hand, the compression members suppress the contraction of the plastic support, thereby functioning to restrain loss characteristics. Further, after being formed into a cable, when the cable and a repeater disposed at the bottom of sea are secured to each other, the compression members function to hold back the optical fiber unit to the housing of the repeater.
The resin filling the space of the plastic support functions to prevent seawater from entering the cable over the whole length of thereof even if a cable accident occurs at the bottom of the sea. Consequently, it is not necessary to replace the whole length of the cable, whereby the damage can be minimized.
A first method of making the optical fiber unit according to the first embodiment comprises an extrusion-molding step of extrusion-molding a plastic support having one or more rigid compression members embedded therein along a longitudinally extending space; and an introducing step of introducing into the space of the plastic support one or more coated optical fibers and a resin having such a viscosity that the coated optical fibers are radially movable therein. Here, the extrusion-molding step and the introducing step are carried out simultaneously.
According to such a method, the plastic support accommodating coated optical fibers is obtained by extrusion-molding a plastic resin. Consequently, as compared with the second conventional example produced by welding, this method can make the optical fiber unit at a higher speed, thus yielding a better productivity. Further, since the extrusion temperature of the plastic support is much lower than the welding temperature, the risk of thermal damage to the coated optical fibers accommodated within the space of the plastic support is lower, whereby this method has a higher reliability. Also, since the introducing step and the extrusion-molding step are carried out at the same time, this method can continuously make a long cable.
A second method of making the optical fiber unit according to the first embodiment comprises a first step of extrusion-molding and taking up a cylindrical plastic support having one or more rigid compression members embedded therein along a longitudinally extending space; and a second step of longitudinally cutting an outer wall of the plastic support defining the space of the plastic support and, while widening the cutout portion thus formed, introducing one or more coated optical fibers and a resin in which the coated optical fibers are radially movable.
In accordance with such a method, the extruding step of extrusion-molding the plastic support and the introducing step of introducing the coated optical fibers and the resin into the space of the plastic support are separated from each other. Therefore, even if a trouble occurs in the step of introducing the coated optical fibers and the like, the manufacture line can be stopped, adjusted, and then restarted. In particular, when a long cable such as a submarine optical cable is to be made, a delicate step of introducing coated optical fibers into a space of a plastic support is made thus stoppable so as not to produce a defective product.
While an optical fiber unit according to a second embodiment comprises a structure basically similar to that of the first embodiment, it further comprises a plastic tube accommodated within the space of the plastic support. A plurality of coated optical fibers are accommodated within this plastic tube, and a gap between an inner wall of the plastic tube and the accommodated coated optical fibers is filled with a waterproof jelly- or gel-like resin. In the optical fiber unit according to the second embodiment, as with the first embodiment, one or more rigid compression members are embedded along the space between the inner wall face of the plastic support and an outer wall face of the plastic support defining an outer shape of the plastic support.
Here, in each of the first and second embodiments, the resin filling the space of the plastic support or filling the plastic tube preferably has such a viscosity that a penetration number within the range of 80 to 1000 is yielded. It is because of the fact that, if the filling resin is too hard, then the coated optical fibers accommodated within the optical fiber unit meanders when the optical fiber unit is bent. This meandering of the coated optical fibers causes the transmission loss to increase.
Further, among the compression members embedded in the plastic support, at least a pair of selected compression members are disposed so as to sandwich the space or the plastic tube disposed within the space.
As at least a pair of compression members are thus disposed at positions opposing each other across the center axis of the plastic support, the thermal contraction force and structure of the plastic support become laterally uniform. This structure restrains the plastic support from twisting at the time when the optical fiber unit is bent.
Also, it is preferred that a plurality of compression members be disposed at each of positions opposing each other across the center axis of the plastic support. As a pair of groups each constituted by a plurality of compression members sandwich the space accommodating the coated optical fibers and the like, the outside diameter per compression member can be reduced. As a consequence, the minimum thickness of the plastic tube and that of the plastic support covering the plastic tube can be enhanced, whereby problems such as projections of the embedded compression members from the plastic support can be reduced. If the minimum thickness of each of the above-mentioned plastic members is held constant, then the outside diameter of the optical fiber unit can be reduced. Such a reduction of the diameter of the optical fiber unit can lower the total amount of the sheath material disposed at the outer periphery of the optical fiber unit, and enables the take-up drum to decrease its size. Also, a longer submarine optical cable can be taken up.
The submarine optical cable comprises an optical fiber unit having such a structure as those mentioned above, a plurality of tension members, and a waterproof structure. The plurality of tension members are disposed around the optical fiber unit. The waterproof structure is obtained by covering the optical fiber unit and the plurality of tension members with a resin layer. As the outer periphery of the optical fiber unit is provided with such an outer sheath structure, a wide-band, low-loss, submarine optical cable is obtained.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.